In conventional hard disk drives and the like, a rotating memory media is divided into circumferential tracks. Data is written to and read from the tracks by read and write heads. The location of the heads is controlled using servo information which is written on the disk at the time of manufacture, through a formatting process.
Servo information typically includes several wedges that extend radially from the center of the disk, across all of the tracks. The servo wedges are only used for tracking purposes, not data recording purposes.
For recording purposes, the tracks are divided into sectors. Today's sectors each typically store 512 bytes of user data. For processing purposes, the bytes are often grouped into 10-bit symbols. The symbol length is determined by the encoding method used by the read/write channel, and error correction code ECC.
The lineal length of all of the sectors is substantially equal, but the lineal distance between servo wedges changes constantly, because the wedges extend radially from the center of the media. As a result, a portion of a sector or a number of sectors can be recorded between adjacent servo wedges. In fact, a sector can be divided into two parts, one on either side of a servo wedge.
This conventional arrangement records data on the tracks in a fairly efficient manner, but if even a small defect is detected in a sector in the servo writing or other formatting process, the entire sector is not used. This is an inefficient use of disk space. Accordingly, there is a need for methods and apparatus for more efficiently isolating defects in memory media when the media is formatted, without isolating an entire sector in which a defect is found.